We use UML2 in several (research) projects and it works for most purposes but at the moment we are stuck with code generation for UML2. Basically there is a code generation for UML2 but it is not complete and only works for packages and elements that are used by class diagrams.
We would like to add more diagram types to the code generation but at the moment we are a stuck doing so. The most important question is "where to start?". Is there any documentation available on how the UML code generation is done so far? Which Eclipse projects(from git) are involved? Or does one have some good advice?

We would like to add more diagram types to the code generation but at the moment we are a stuck doing so

I am not sure I understand what you mean. Could you expand on that?

The code generation used to implement the UML metamodel in the Eclipse UML2 project is a feature of the EMF project (which IIRC is based on an older version of the JET code generation tool, also an Eclipse project). That feature is not UML-specific, and works with any EMF-based models. You should be able to get better help on that in the EMF and (for the newer JET version) M2T newsgroups. You will also find other code generation solutions from Eclipse.org in that last newsgroup.